High strength investment casting mold



Sept. 16, 1958 H. L. BENHAM HIGH STRENGTH INVESTMENT CASTING MOLD Filed April 8, 1957 INVENTOR.

I a a o I T. n n a I .o a a. a 'o 0 U IO 0 a c a a a A .ixfigammfim.

ATTORNEY United States Patent HIGH STRENGTH INVESTIVIENT CASTING MOLD Harold L. Benham, Redford, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application April 8, 1957, Serial No. 651,273

11 Claims. (Cl. 22-193) This invention relates to a high-strength investment mold for casting metal and particularly to an improved investment molding composition which produces a mold having generally uniformly distributed refractory particles.

In precision casting metal articles by means of refractory molds it is frequently necessary that the molds possess high strength and exceptional surface smoothness. Thus, in investment molding it is usually desirable to form the principal refractory portion of the mold of a hard material to which a coating on the invested pattern will tightly adhere, thereby providing the mold with a smooth casting surface upon removal of the destructible pattern. The mold must have suflicient strength and rigidity to prevent cracking of the mold during the burnout or firing operation and to withstand the pressure of the molten casting metal. This is especially important when the investment mold is to be used for producing hollow castings such as turbine blades having cored cavities or passages of small cross-section. Of course, the mold also should possess satisfactory porosity, and the surface and setting characteristics of the investment material should enable it to bond tightly to the pattern coating.

Many investment molds heretofore used for casting hollow turbine buckets or the like cracked during the firing operation, thereby producing defective castings. This cracking resulted from differences in thermal expansion between refractory constituents in the mold because these constituents were not properly distributed in the molding mix while the binder was setting.

Accordingly, a principal object of the present invention is to provide an improved investment composition for forming high strength refractory molds to be used in precision casting operations. A further object of this invention is to provide a refractory mold and a process for forming the same in which the refractory particles in the investment material are properly distributed and prevent cracking or distortion of the mold during burnout of the pattern and firing of the mold.

These and other objects are attained in accordance with this invention with an investment material consisting essentially of a mixture of a dry mix or grog, a small amount of a metal stearate, and an ethyl silicate binder. The grog preferably contains a pulverized fire clay, a fine refractory filler material, and an accelerator for the binder. Borax glass or cryolite also may be included in the investment mixture to further increase the hightemperature bond and toughness of the mold. After firtypes of high-temperature alloy turbine blades. However, in certain applications such as casting hollow turbine buckets or nozzle guide vanes in which the invest ment material forms the cored cavities, it is particularly important that the molds possess high strength and suitable thermal expansion properties since molds of the necessary configuration are especially prone to crack or distort excessively if subjected to undue stresses. These stresses, which are normally produced in investment molds during the firing operation, result from the manner in which the molds are formed. As will be hereinafter more fully described, the investment mix is poured into a flask around a coated fusible pattern. The flask, pat tern and molding mix are then rapidly vibrated in order to eliminate any air trapped in the mix and to cause the mix to intimately contact all pattern surfaces. This is particularly important in forming molds for casting hollow turbine blades because the molding mix must completely fill the hollow or cored interior of the pattern. Since this cored passage may be rather long and of very small cross-section, the investment mix must be vibrated .for an appreciable period of time to insure proper settling of the mix.

The investment mix compositions previously used do not always produce satisfactory results, however, because all the refractory constituents in the mix tend to settle or slump simultaneously during vibration of the flask. As a result, these refractories bridge over to form a refractory head which entraps liquid binder and air bubbles within the hollow pattern. This mechanism is especially noticeable in relatively tall molds, such as are frequently employed in casting many types of hollow turbine blades. As vibration continues, the refractory head further sinks or settles in the flask. This settling causes a portion of the entrapped binder to move outwardly toward the side walls of the fiask, carrying wit-h it fines of the investment mix. This binder portion and these fines, which are usually silica flour, then rise to the top of the mold and establish a binder head. The fines slowly settle out of this binder head, producing a layer of fines on top of the heavier, coarser refractory portion of the investment mix and beneath the binder head. This partial segregation of the constituents in the molding mixture results in relatively loose packing of the refractory particles, entrapment of portions of the liquid binder within the mold, and non-uniform distribution of fines throughout the mold. Small air bubbles are also trapped within the molding mix, producing undesirable voids in the mold, particularly at the cored 'area.

When the resultant mold is heated from room temperature to about 1800 F. to 2000 F. during firing of the mold, the mold expands on the average approximately 0.7%. However, this growth is the combination of the thermal expansion of both the fire brick and the silica flour or fines. Unfortunately, the silica flour expands approximately 1.5% during this heating operation while the fire brick expands only about 0.35% to 0.5%. Accordingly, if much of the silica flour has separated from the fire brick during the vibration of the mold, stresses are set up in the mold due to the differential expansion of the two refractory constituents. These stresses produce mold cracks which result in flash on the castings and generally poor dimensional quality of the castings. Moreover, the burning out of the mold causes voids to be formed in the trapped binder areas. At these locations the investment back-up for the dip coat layer is inadequate, and the casting produced in such .a mold h'as excess metal in its cored area and blister-like defects on its surfaces. Of course, the mold is also weaker and tends to crack or distort because the refractory grains are not tightly packed. A hollow turbine blade cast in a mold of this type not only has improper dimensions, but it frequently ha-s refractory material embedded in its surfaces.

On the other hand, when a small amount of magnesium stearate powder is included in the investment molding mix in accordance with the present invention, the aforementioned difiiculties are circumvented. The magnesium stearate changes the suspending properties of the binder so that the refractory constituents in the mix settle in a difl erent manner when the mix is poured into the flask and subsequently vibrated. In general, these refractories are much more uniformly suspended than when magneslum stearate is omitted. Upon vibration of the flask and molding mix, the particles of fire brick and silica flour remain properly suspended and hence tend to pack uniformly within the flask. These refractory constituents slowly and uniformly pack upwardly from the bottom of the flask until finally the refractory head portion of the mold above the pattern also becomes an integral part of the refractory mold body. As a result, the binder head or area of high liquid binder concentration above the refractory head contains less fines than molds formed in conventional manner. The size of the intermediate layer containing a large proportion of fines is accordingly reduced to a substantial extent.

An investment molding mix of the composition described herein produces a mold containing a minimum of entrapped and segregated binder. This is especially important at areas adjacent the pattern dip coat layer and in the cored or hollow areas of the pattern. As indicated above, the investment material also is more uniformly distributed throughout the mold body and the refractory particles are more closely packed. The mold thus formed has proper density and high strength because of the decrease in the number of voids which are otherwise caused by entrapped air and liquid binder. Such a mold substantially eliminates the formation of lumps of blister-like excess metal on the outer surfaces and in the cored areas of investment castings. Moreover, there is far less tendency for the mold to crack during the firing operation because it expands more uniformly. Consequently, the amount of flash on castings is reduced and better dimensional control of the castings is maintained.

Other objects and advantages of this invention will more fully appear from the following detailed description thereof taken in conjunction with the accompanying drawing, which contains a somewhat schematic sectional View of a destructible pattern invested in a refractory mold formed in accordance with the invention.

Referring more particularly to the drawing, a pattern of a hollow turbine bucket to be cast is shown invested in a refractory mold 12 within a metal container or flask 14 positioned on a base plate 16. The pattern 10 is preferably formed of a low fusing substance, such as wax or a thermoplastic resin, or any other vaporizable, fusible, combustible or otherwise readily destructible material. Among the plastic patterns which have been found to be satisfactory are those formed of polystyrene, although other thermoplastic pattern materials, such as resinous polymerized derivatives of acrylic acid and resinous polymerized derivatives of methacrylic acid frequently may be used.

The pattern may be formed in a conventional manner by injection molding. The particular pattern shown in the drawing consists of two sections: an elongated, slender turbine blade-forming portion 18 and a lower portion 20 which forms the sprue or pouring basin for the blade casting. The former is provided with a cored area or passage 22 of small cross-section which extends through its entire length.

After the pattern sections are molded, they are preferably cleaned with an alcohol solution and air dried prior to assembly. The two sections of the pattern may be joined together with molten wax or connected by suitable mechanical means. Before the assembled pattern is invested in the mold, it is provided with a surface layer of an appropriate coating material 24 which is to form the casting surfaces of the refractory mold. This coating material may comprise an aqueous dispersion of conventional finely comminuted refractory materials, a binder such as an air-setting silicate cement, and defoaming and wetting agents. Gelatine and acids can also be advantageously included in the coating mixture. This type of coating is dis-closed in United States Letters Patent No. 2,752,257, which issued on June 26, 1956, in the names of James P. Bradley and Robert R. Dohrmann.

Coating of the pattern 10 is preferably accomplished by dipping the pattern in the coating solution. Although in some instances the coating may also be applied by spraying or painting it on the pattern or in any other suitable manner, dipping is preferred because it assures more uniform coating of all of the pattern surfaces and is the simplest method of application.

The dip coat slurry is preferably kept in constant motion by stirring means except during the actual dipping operation. However, the mixing action should not be such as to unnecessarily introduce air into the slurry. The destructible pattern used for forming turbine buckets or other precision cast parts is immersed in the dip coat slurry, preferably to within an inch or so of the outer end of the pouring basin portion 20 of the pattern. Care should be exercised in immersing the pattern in the slurry to prevent air entrapment on pattern surfaces. This is particularly important when coating a hollow pattern of the type shown in the drawing because it is essential to coat all the surfaces 26 which define the passage 22 in the pattern. Normally the dip coat solution is retained at room temperature during the dipping operation because excessive heat can result in distortion of the plastic pattern. The surplus coating material is permitted to drain oft prior to subsequent treatment and investment.

After the pattern has been completely coated with the dip coat slurry, it may be sanded or stuccoed to provide a rough surface on the coating, thus insuring greater adhesion between the principal refractory portion 12 of the mold and the dip coat 24 on the pattern. This sanding may be accomplished by merely screening or otherwise applying silica or other suitable comminuted refractory materials in known manner to the outer coated surface of the destructible pattern. When all the molding surfaces of the pattern have been effectively covered with sand, the pattern should be air dried.

When the pattern has been prepared in the abovedescribed manner, the investment material 12 is formed about both the blade-defining portion 18 and the sprue portion 20 of the pattern. The sprue portion extends through the lower wall of the resultant refractory mold so as to permit the escape of the destructible pattern material and to form an ingate for the molten casting metal. This main refractory mold may be formed about the pattern in any suitable manner, but the following procedure provides excellent results. The base plate 16 is preferably first sprayed or otherwise coated with molten wax so as to form a thin layer 28 of wax on the upper surface of the plate. Before the wax is completely solidified, the pattern to be invested is positioned on the wax-coated plate 16 with the pouring basin portion 20 thereof extending downwardly and seated firmly in the wax film. The sleeve or flask 14 is next placed around the pattern and pressed lightly into the wax layer. In order to completely seal the flask 14 to the plate 16, it is preferable to again spray or pour molten wax over the outer surfaces of these parts at their junction 30. The wax in the resultant assembly should be allowed to thoroughly solidify before proceeding further.

After the refractory mixture, which will be hereinafter described in detail, has been mixed with a proper amount of the liquid binder, it is poured into the sleeve or flask 14. The flask is preferably vibrated during this pouring operation, and the mold is then allowed to set. It will be noted that the pattern sections 18 and 20 are spatially 'tern.

.5 separated at 32 to permit the lower end of the passage 22 to freely communicate with the investment material 12. With this construction, vibration of the flask causes the investment mix to completely fill the passage 22 and form an elongated, narrow core therein. This core extends the entire length of the blade portion of the turbine bucket to the cast.

When the mold body has solidified or set to a suflicient extent, the base plate 16 is removed from beneath the mold and heat is applied to melt the pattern. It is necessary to apply sufficient heat to raise the mold temperature above the fusing point of the pattern material, thus allowing it to escape through the sprue opening in the mold formed by the pattern portion 20. In this manner the dip coat which had covered the surfaces of the pattern tightly adheres to the mold and provides the casting cavity with a smooth coating. It is also possible to vaporize the pattern, if a vaporizable material is used, by heating the mold rapidly to a high temperature.

After removal of the pattern from the mold in the foregoing manner, the mold is fired or burned out, usually at a temperature of about 1800 F. to 2000" F., to remove substantially all the volatile matter. The mold is then preferably preheated to the desired temperature, and the molten casting metal is poured or otherwise introduced into the mold cavity formed by the pat- In the majority of instances it is necessary to pour the casting metal while the mold is still hot in order to obtain satisfactory results. After the molten metal has been poured and the casting has solidified, the refractory mold body 12 and the adhering coating 24 may be broken to permit the removal of the casting. The finished casting possesses excellent surface smoothness and detail and requires little finishing.

The investment material used to form the body or principal refractory portion 12 of the mold in accordance with the present-invention consists of a dry mix or grog, a small amount of magnesium stearate, and an ethyl silicate binder. The dry mix comprises major proportions of a pulverized, fire clay, such as dead burned fire clay or fire brick, and a finely comminuted refractory material and a minor proportion of an accelerator or gelation agent for the binder. Magnesium oxide is the wetting accelerator preferably employed, but magnesium carbonate, calcium carbonate, sodium carbonate, and other alkaline oxides or carbonates may be used. In order to increase the high-temperature bond of the resultant mold, borax glass or cryolite also may be included in the mix. The binder for the grog consists of an aqueous solution of condensed ethyl silicate, alcohol and an acid. Denatured ethanol of approximately 190 proof spirit is the alcohol preferably employed, while concentrated hydrochloric acid is the acid which has been found most desirable to add to the binder solution.

Accordingly, a satisfactory investment dry mix or grog is one comprising, by weight, approximately 65% to 90% of the finely ground burned fire clay or brick, 9% to 34% silica flour or other finely comminuted refractory material, and 0.15% to 1.5% of an accelerator or gelation agent, such as magnesium oxide, for the binder. In order to provide optimum results, the fineness of this mix should be between about 90 and 100 A. F. A. An accelerator content of approximately 0.2% to 1% is preferred for most applications, the greater the amount of accelerator added the lesser the gel time of the mold produced. When borax glass or cryolite is included in the grog to further improve the high-temperature bond of the resultant mold, either constituent may be present in an amount not in excess of about 2% by Weight. In general, it is advantageous to employ at least 0.1% of such a high-temperature bonding agent. The preferred borax glass content of the dry mix is 0.3% to 1%, while a dry mix containing 0.5% to 1% cryolite appears to provide optimum results.

After the dry mix or grog has been prepared, a small amount of water is added to it and the resultant mixture mulled for a short period of time. A mulling period of approximately 30 seconds to 1% minutes has proved to be satisfactory. Approximately to 150 cc. of water is all that is necessary for most applications, this amount being equivalent to about 0.2% to 0.6% of the weight of the grog.

Next the magnesium stearate is added, about one pound of magnesium stearate to 700 pounds of grog being preferred for many applications. The exact amount of stearate to be used depends to a considerable extent upon the dimensions and configuration of the casting it is desired to form. Generally the magnesium stearate should constitute only about 0.04% to 0.7% of the total weight of the grog. However, I have found that the optimum magnesium stearate content of the grog is between 0.1% and 0.3% by weight. It is desirable to mull the mix for about two to five minutes, depending on the size of the batch, after the magnesium stearate has been added.

As hereinbefore pointed out, the use of magnesium stearate in accordance with this invention causes the refractory particles in the investment molding mix to be much more uniformly distributed in the mold. The resultant improved packing of the mold material substantially decreases the amount of refractory fines contained in the binder head 34. Moreover, the size of the layer 36, which is located between the principal portion of the refractory mold body and which has a high concentration of fines, is greatly reduced. Of course, relative dimensions of various parts of the mold assembly are exaggerated in the drawing in order to more clearly show the relationship of these parts.

Although magnesium stearate provides the most satisfactory results, other metal stearates may be partially or wholly substituted for it to beneficially affect the packing of the refractory particles in the mold. Various light metal stearates and alkaline metal stearates, such as calcium stearate, sodium stearate, potassium stearate and aluminum stearate appear to be useful for this purpose.

The aforementioned ethyl silicate type of binder preferably comprises, by weight, approximately 35% to condensed ethyl silicate, 35% to 60% alcohol, 0.1% to 0.4% concentrated hydrochloric acid and 5% to 13% water. Depending on the particular application, the ratio, on a Weight basis, of the dry mix or grog to the ethyl silicate binder may vary from about 2.5 to 4.5. When the above investment dry mix and the binder are mixed in a proper ratio, the resultant investment material generally comprises, by Weight, approximately 43% to 74% pulverized fire clay or brick, 6% to 25% silica flour, 0.03% to 0.6% magnesium stearate, 0.1% to 1.2% magnesium oxide, 6% to 17% condensed ethyl silicate, to 17% alcohol, 0.02% to 0.1% hydrochloric acid and 0.9% to 3.7% water. A sufficient amount of additional water may be added during mixing of the grog and binder to raise the water content to as high as 7%.

For optimum results the burned fire clay content should be between 55% and of the weight of the entire mixture, this amount being equivalent to approximately 70% to 87% of the dry mix or grog. If borax glass or cryolite is included in the mixture, the former preferably constitutes between 0.6% and 1.4% of the weight of the final investment material while a cryolite content of 0.1% to 1.2% is appropriate. Most satisfactory molds with respect to high strength and freedom from cracks are produced when the final investment mix contains about 0.08% to 0.2% by weight of magnesium stearate.

The finely ground dead burned fire clay or fire brick functions as the refractory base of the investment mixture and must be selected so that it has desirable expansion characteristics. This material should also be of uniform quality and composition and reasonably free of foreign matter. Calmo is an example of such a pulverized burned fire clay. Various mixtures of relatively coarse and fine Calmo fire clays may be used, these clays varying in A. F. A. finenesses from approximately 20 to 120.

Silica flour is preferably added to increase the fines in the refractory base and to eliminate voids in the backing of the mold. Other refractory powders, such as zirconium silicate or zirconium oxide flour, may be used for particular applications, however. Generally, it is desirable to use a silica flour which is fine enough to permit at least 99% of it to pass thorugh a 140 mesh screen.

The magnesium oxide is used in powdered form and may consist of either heavy or light magnesium oxide, or mixtures of these oxides. It is preferable that this ma terial be of sufiicient fineness so that at least 90% of it will pass through a 200 mesh screen. Likewise, borax glass or cryolite, if employed as a secondary high-temperature bonding agent in the investment mix, should be added in powder form, preferably of approximately 90 to 110 mesh.

The condensed ethyl silicate in the investment binder, of course, is a source of silica for the reaction in which ethyl silicate and water form silica gel and alcohol. Upon drying, silica is the ultimate primary binder for the investment. Alcohol is included to produce mutual solubility of ethyl silicate and water since these latter constituents are immiscible in the absence of the alcohol. The hydrochloric acid is necessary for pH control and to regulate the speed of the aforementioned reaction and the subsequent mold gelation.

It is preferable to use a condensed ethyl silicate having not less than 25 available silica as SiO An example of such an ethyl silicate is one consisting of approximately 85% tetraethyl orthosilicate and 15% polysilicates. Satisfactory commercially available products of this composition frequently have 0.1% maximum acidity as hydrochloric acid, a flash point (open cup) of approximately 90 F., and a specific gravity between 0.920 and 0.950 at 20 C. Hence a typical example of the ethyl silicate solution preferably used is a condensed ethyl silicate which contains, by volume, approximately 50% ethyl silicate, 0.1% hydrochloric acid, and the balance alcohol and water. It will be appreciated, however, that any hydrolyzed or condensed ethyl silicate solution may be satisfactorily used so long as the ethyl silicate content is sufficient to provide the proper bonding properties.

With respect to the permissible upper limit of the concentration of the ethyl silicate solution, it is desirable that condensing conditions exist. Accordingly, to ensure these conditions, it is feasible to use a solution which contains only enough water to provide a sufiicient amount of hydrolysis to obtain the above satisfactory bonding and strengthening effects, or to otherwise use ethyl silicate under hydrolyzing conditions. Thus, excellent results are obtained by using a mixture containing hydroylzed or condensed ethyl silicate solution with an ethyl silicate content between 25% and 75% by volume.

In preparing the binder solution, the water and acid are first mixed together, and the alcohol and condensed ethyl silicate are subsequently added. The solution is then stirred and permitted to set for several hours prior to use. A preferred method of mixing the grog and liquid binder consists of placing the dry mix or grog, together with the secondary water equal to about 2% of the weight of the binder, in a rotating batch mixer. The magnesium stearate powder is then added, and the mixture is mulled for approximately three minutes. Next the binder solution is introduced and the resultant slurry mixed for about ten minutes. Although it is preferable to follow this procedure, in some instances satisfactory results can be obtained by first mixing the dry grog and binder solution and thereafter adding the magnesium stearate.

The above-described refractory investment mold is particularly well adapted for use in casting hollow articles having curved surfaces, such as hollow turbine blades, for

reasons hereinbefore explained. Furthermore, this mold does not detrimentally react with nickel base alloys and cobalt base alloys, materials commonly used for cast turbine blades, and hence has no adverse effects on the surface qualities of such blades.

While the present invention has been described by means of certain specific examples, it is to be understood that the scope of the invention is not to be limited thereby except as defined in the following claims.

I claim:

1. In an investment mold composition consisting essentially of comminuted refractory material and a silicate-type binder, the improvement which consists of including therewith magnesium stearate in an amount equal to about 0.04% to 0.7% of the weight of said refractory material.

2. A mold composition consisting essentially of a mixture of an ethyl silicate solution and a grog comprising, by weight, approximately 65% to of a pulverized fire clay, 9% to 34% of a finely comminuted refractory material, 0.04% to 0.7% metal stearate, and 0.15% to 1.5% of a setting accelerator for the ethyl silicate solution.

3. A highly permeable refractory mold having a body portion resulting from setting of a mixture of an ethyl silicate solution and a grog comprising, by weight, approximately 65% to 90% of a finely pulverized fire clay, 9% to 34% silica flour, 0.04% to 0.7% magnesium stearate, and 0.2% to 1% of a setting accelerator for ethyl silicate, the ratio, by weight, of the grog to the ethyl silicate solution being between 2.5 to 1 and 4.5 to l.

4. A mold composition consisting essentially of a mixture of a grog comprising, by weight, approximately 65% to 90% of a pulverized burned fire clay, 9% to 34% D of a finely comminuted refractory material, 0.04% to 0.7% powdered magnesium stearate, 0.15% to 1.5% of a setting accelerator for ethyl silicate, and an ethyl silicate binder solution comprising, by weight, about 35% to, 60% condensed ethyl silicate, 35 to 60% alcohol, 0.1% to 0.4% of an acid, and 5% to 13% water.

5. An investment composition for a highly permeable refractory mold consisting essentially of a mixture comprising, by weight, approximately 70% to 87% of a finely ground dead burned fire clay, 9% to 34% silica flour, 0.1% to 0.3% powdered magnesium stearate, and 0.2% to 1% magnesium oxide and an ethyl silicate binder solution comprising, by weight, about 35% to 60% condensed ethyl silicate, 35% to 60% alcohol, 0.1% to 0.4% concentrated hydrochloric acid and 5% to 13% Water.

6. A refractory mold characterized by high permeability, said mold comprising an investment body portion formed of the residue of a mixture consisting, by weight, essentially of approximately 43% to 74% of a pulverized fire clay, 6% to 25 of a finely comminuted refractory material, 0.03% to 0.6% of a metal stearate selected from the class consisting of magnesium stearate, calcium stearate, sodium stearate, potassium stearate and aluminum stearate, 6% to 17% condensed ethyl silicate, 6% to 17% alcohol, 0.02% to 0.1% of an acid, 0.9% to 7% water, and 0.1% to 1.2% of a setting accelerator for the condensed ethyl silicate.

7. An investment composition for a highly permeable refractory mold comprising, by weight, approximately 55% to 70% of a finely pulverized dead burned fire clay, 6% to 25% silica flour, 0.08% to 0.2% powdered magnesium stearate, 0.1% to 1.2% magnesium oxide, 0.1% to 2% powdered cryolite, 6% to 17% denatured ethyl alcohol, 0.02% to 0.1% concentrated hydrochloric acid, 0.9% to 3.7% water, and 6% to 17% condensed ethyl silicate solution, the ethyl silicate content in said solution being between 25 and 75 by volume.

8. A method of forming a refractory mold which comprises coating a readily destructible pattern with a refractory coating mixture, investing said coated pattern in a refractory molding mix consisting essentially of a grog comprising, by Weight, approximately 65% to 90% of a finely ground burned fire clay, 9% to 34% silica flour, 0.04% to 0.7% magnesium stearate, and 0.15% to 1.5% of a setting accelerator for ethyl silicate, and a binder for said grog comprising a solution of approximately 35% to 60% condensed ethyl silicate, 35% to 60% alcohol, 0.1% to 0.4% of an acid and 5% to 13% water, and thereafter eliminating the pattern from said mold, where'- by said coating adheres to the refractory mold.

9. The process of forming a highly permeable refractory mold having a smooth casting surface, said process comprising applying a refractory coating to a fusible pattern, drying said coating, thereafter investing said coated pattern in an investment material comprising, by weight, approximately 43% to 74% of a finelypulverized dead burned fire brick, 6% to 25% silica flour, 0.1% to 0.3% powdered magnesium stearate, 0.1% to 1.2% magnesium oxide, a small but etfective amount of borax glass not in excess of 2%, 6% to 17% condensed ethyl silicate, 6%

to 17% ethyl alcohol, 0.02% to 0.1% concentrated hy- 20 drochloric acid and 0.9% to 7% water, setting the investment material, melting and removing the pattern from the formed mold, whereby the coating tightly adheres to the walls of the casting cavity of the investment material, and thereafter heating said mold at a temperature suflicient to remove substantially all of the volatile and combustible matter therefrom.

10. In an investment mold composition consisting essentially of comminuted refractory material and a silicatetype binder, the improvement which consists of including therewith a metal stearate in an amount equal to about 0.03% to 0.6% of the total weight of said composition.

11. In an investment composition for a refractory mold consisting essentially of an ethyl silicate-type binder and an investment grog consisting essentially of pulverized fire clay, a finely comminuted refractory material, and a gelation agent for said hinder, the improvement which consists of including therewith metal stearate in an amount equal to about 0.04% to 0.7% of the weight of said grog.

References Cited in the file of this patent UNITED STATES PATENTS 1,906,357 Beckman May 2, 1933 2,441,695 Feagin et al May 18, 1948 2,683,296 Drumm et al July 13, 1953 FOREIGN PATENTS 692,030 Great Britain May 27, 1953 753,228 Great Britain July 18, 1956 

1. IN AN INVESTMENT MOLD COMPOSITION CONSISTING ESSENTIALLY OF COMMINUTED REFRACTORY MATERIAL AND A SILICATE-TYPE BINDER, THE IMPROVEMENT WHICH CONSISTS OF INCLUDING THEREWITH MAGNESIUM STEARATE IN AN AMOUNT EQUAL TO ABOUT 0.04% TO 0.7% OF THE WEIGHT OF SAID REFRACTORY MATERIAL. 